Hip Fracture Epidemiology
Fractures of the proximal femur are a major source of mortality and morbidity among the elderly. Approximately 250,000 hip fractures occur in the United States annually. Nearly 33 percent of women and more than 17 percent of men will experience a hip fracture if they live to age 90. Among patients who are functionally independent prior to a hip fracture, 15 to 25 percent remain in long-term care settings for more than a year afterward. Another 25 to 35 percent are dependent on others for their mobility. More than half of those that survive hip fracture never recover normal function. Moreover, the average mortality associated with hip fracture in elderly patients is approximately 20 percent in the first year.
The public health impact of hip fractures is also staggering. Recent surveys in the United Kingdom have shown that at any one time about 50 percent of acute orthopedic beds are occupied by hip fracture patients. In the U.S., the average length of hospital stay for hip fracture patient is three weeks, longer than for any other diagnosis. The annual costs associated with the acute and chronic care of hip fracture patients in the United States is estimated to exceed $7 billion. The problem can only be expected to worsen with projected increases in the average age of the world population, leading some to suggest the possibility of a nearly three-fold rise in the total number of hip fractures by the middle of the next century.
Over 90 percent of all hip fractures are caused by falls. However, the majority of falls in the elderly result in only minor injury, with one to three percent causing hip fracture. Surveys among elderly fallers have shown the following factors increase the risk for suffering a hip fracture in a fall (in order of importance): 1) impacting on the hip or side of the leg (which increases the risk of fracture by over 20-fold), 2) having a tall, slender body habitus, 3) falling with a high initial potential energy (which depends on both body mass and the height of the fall), and finally 4) possessing low bone density in the proximal femur. These results suggest the risk for hip fracture in a fall is dominated by the severity of the fall as opposed to the density and strength of the proximal femur. To reduce the incidence of hip fractures, hip fracture prevention strategies must therefore either reduce the incidence of falls, or protect the femur in the event of a fall. The former strategy may be accomplished by restricting mobility, although this carries associated medical risks and impairs personal autonomy and quality of life. It might also be accomplished through exercise programs in the elderly populations at greatest risk for falling, although no study to date has proven the effectiveness of such an approach. In any case, it seems unrealistic to expect the complete elimination of falls among the elderly, given the often multiple factors (cardiac, neural, musculoskeletal) and random causes of falls. It therefore appears that the most reliable method for reducing hip fracture incidence is to protect the femur during the impact stage of the fall. This essentially requires lowering the impact force applied to the femur to a value below its fracture threshold.
Hip fracture refers to fracture of the proximal end of the femur, which is the strongest, heaviest, and longest bone in the body, accounting for approximately one-fourth of total body height. The proximal end of the femur consists of a head, neck, and greater and lesser trochanters. The neck of the femur connects the spherical head to the shaft. It is limited laterally by the greater trochanter, a large, somewhat rectangular lateral projection from the neck and shaft, which provides an insertion site for several muscles of the gluteus region. The greater trochanter lies laterally, just beneath a relatively thin layer of skin and adipose tissue (fat), and can be easily palpated on the lateral side of the thigh. Since it is the most lateral point of the hip region, the greater trochanter is the site which comes into contact with a hard surface when one lies on one's side, and the site where the majority of impact force is applied when one falls sideways onto the hip. Consequently, falls to the side resulting in impact to the greater trochanter carry a high risk for hip fracture.
In contrast to the minimal amount of soft tissue covering the greater trochanter, a considerable quantity of soft tissue exists in the posterior gluteal (buttock) and anterior thigh regions adjacent to the greater trochanter. Upon impact to these regions, this soft tissue is able to absorb significant energy, and lower the impact forces applied to the underlying skeletal structures. Gluteal soft tissues include the gluteus maximus, medius, and minimus muscles, as well as the considerable layer of fat overlying the buttock. The most significant anterior thigh soft tissues are the quadriceps muscles, which include the three vastus muscles and the rectus femoris muscle.
Experimentally, when the elderly cadaveric femur is loaded in a configuration simulating a fall on the hip, the average force required to fracture it is 2040N [Lotz JC and Hayes WC, J Bone Joint Surg [Am], 72-A:689-700, 1990]. The corresponding average energy absorbed by the bone up to fracture is 25 J. At standing height, the potential energy of the body can be well over 20 times this amount. Furthermore, we have conducted experiments which suggest the force applied to the femur at impact from an average sideways fall to the hip is about 6 kN, over three times the mean fracture force (Robinovitch, SN, Hayes, WC, McMahon, TA, J Biomech Eng, 113: 366-374, 1991). It therefore appears that to avoid hip fracture during a fall, one or more of the following must occur: 1) direct impact to the lateral aspect of the hip must be avoided, 2) the impact site must extend outside the hip region, or 3) significant energy must be absorbed by alternative mechanisms such as contraction of the thigh muscles during descent, breaking the fall with an outstretched hand, or deformation of both the floor and the soft tissue overlying the impact sight.
Similar to hip fracture, fracture of other bones such as the tibia, radius, and ulna occurs when the force applied to the bone exceeds that required to initiate fracture. Often such a situation arises when the impact energy is high, and contact occurs to a small area directly overlying the fracturing bony structure. In such circumstances, the impact energy cannot be absorbed and/or dissipated through a large area, and high local stresses are applied to the underlying bone.